Process of case hardening



Patented July 7, 1931' UNITED STATES ROYSEL J'. COWAN AND PHILLY W. MCCOY, OF TOLEDO, OHIO, ASSIGNORS TO SURFACE COMBUSTION CORPORATION, OF TOLEDO, OHIO, A CORPORATION OF NEW YORK PROCESS OF CASE HARDENING No Drawing.

This invention relates to improvements in i the art of case-hardening metal objects by a gaseous medium more particularly ammonia gas and has for its objects to provide a continuous process of case hardening as distinguished from the usual intermittent or batch type process; to provide a process of case hardening that shall permit more complete utilization of the case-hardening constituent of the gaseous medium, and withal to provide a process which shall give a more uniform case than can be obtained by the batch type process heretofore used.

Surface hardening of metals by ammonia gas is known as nitriding and nitriding processes as heretofore conducted are fully set forth in the publication entitled N itriding Symposium being the Transactions of the American Society for Steel Treating, Vol. XVI, October, 1929, No. 5, published by American Society for Steel Treating, Cleveland, Ohio.

As will clearly appear from the above entitled publication, nitriding as heretofore conducted is a batch type process. That is to say the articles to be nitrided are placed in a suitable container adapted to be placed within a suitable heating chamber without being moved through the chamber and ammonia gas passed through'the container during the heating operation. The more usual type of equipment for carrying on the nitriding operation consists of a gas tight box wherein the articles can be supported on a screen, the inlet tube for ammonia gas extend-' ing along the bottom of the box below the screen while the outlet is at the front end and near the top of the box. (See page 7 4 of said publication.) The box type process, however, givesgnore or less erratic results and it has, therefore, been proposed to provide means such as afan for agitating or stirring u the nitriding gases in the work container. (See page 150 of said publication.)

The time of nitriding with processes heretofore used varies with the individual meth-' od and purpose from 18 to 90 hours and the usual temperature range is from 850 to 1150 F. and the ammonia dissociation is usually kept Within to 40 percent.

Application filed March 8, 1930. Serial No. 434,445.

Now we have discovered and this forms the basis of our invention that nitriding can be carried on at the usual temperatures as a continuous process, that is to say, where the articles are passed in continuous course through a heating chamber, with more uniform results and with a higher degree of dissociation of the ammonia gas and consequently with the use of much less ammonia and in appreciably less time than heretofore, provided the ammonia gas and the work to be nitrided have unidirectional flow as distinguished from counterflow through the. heating chamber. As previously stated the usual or permissible limits of ammonia dissociation with prior processes is between 30 to 40 percent. With our process the ammonia dissociation may be practically complete for example 90 percent or more at the outlet from the heating chamber. The heating chamber may be of any suitable form, as for example, a tunnel type mnfile chamber provided with suitable .means at its ends permitting entry of work into and discharge of work from the heating chamber without inflow of air into the chamber. The work may be moved through the heating chamber in any suitable manner, as for instance, small articles may be placed in wire baskets adapted to be pushed through the heating chamber or a conveyer belt may be provided for supporting the work. The main thing as already stated is that the gas and work shall flow in the same direction. As

already indicated the heat-treating temperatures will follow the usual practice. It will, of course, also be understood that the ammonia gas will be introduced into the heating chamber at the work-entering end of the chamber and will leave at the opposite end and that the chamber will contain a plenum ofthe gas. The gas will. of course, be introduced into the heating chamber under sufficient pressure to cause a flow of the gas through the chamber, the flow being so regulated to obtain as high degree of dissociation at ihe outlet from the chambers as may be desired.

As already indicated dissociation may proceed to a degree approaching complete dissovent'on except as may fbe expressed the 9: .to be ,nitrided and ;moni a; is prevented the case hardening taking P ce-pr g ess v ly wi h diil c e io e the ammonla, as. Our-experlments have shown that high issociation of the ammonia gas will;

not impair a nitrided case When once formed,

but on the-contrary actually'improves-the quality of the case thereby producing a better jcase inthe samelength of time than is produced in the intermittent process. It

vvilLth'erefore; bereadily appreciated that Lamaze chamber and simultaneously passing am-. monia gas through said chamber in the same direction'of flow as said objects.

5. A process of nitriding permitting obtainment of suitable case with ammonia dissociation approaching com lete dissociation gcoinprising passing the "o jects 1 through a heating chamber and'introdiic-ing into said chamber for flow in the same direction as said objectsaregulated stream of ammonia I testimony'whereofweaflixoursi atures.

i ROYSEL J. Cog AN.

PHILIP WM. MCCOY.

according toiourinvention; the products of dissociation of' the ammonia gas are belng continually removed from the; work by the 2 ercisfg any decarburizing' effect. However,

Jme s @01 lusively; show that a superior ni- I 'wha ver y e fl ftr k plan o l dfthe i eaction whichtakesplace by causing the ami monia gas andwork' to flow' in the same dijrectjion; th factremains' thatfou'r experitrifd'edcase' canfbe produced in a'shorter time if heretofore proposed. f

' "Ou'rexpe'r'iments sho jsults are obtained b introducing an of the and vwith lessammonia by any process 1 that satisfactory reammoniaat the charging endof the furnace or: mufiiebut it iswithin the spirit of the inventionto al'so'introduce ammonia at points between-the ends of the furnace.

i utility where the case hardening medium is ammoniagas, nevertheless it also has appli-' cation withother casehardenmg gases and, Y

thereforaitis not intended to .limitthe inapp'ended claims "j What: is claimed is": a I I lfAjpi ocessof case hardeningobjects'by ammo a; 'g as comprisin tional flow through a heating chamben" i 23111 a process of case hardening objects:

where the obj cots" are moving incontinuous course throu'gh'a heating -chan1ber,-J-the step of passing animonia gas through said cham-* ber 'inthe same'direction of flow as said objects. I contlnuous processof mtnd ng com- 3. A prising simultaneously, passing the "objects the nitriding gas in unidirectional flow through a heatlng chamber.

:4. A process of nitriding permitting the l' While our improved pro'cess has special simultaneously v passing said obj ects'and saidgas' in unidirecobtainment of suitable case with'ammonia dissociat on 1n excessof 40 percent comprlsing ipassmgfthe 'objec'ts a heating 

